System and method for discovering frequency related spurs in a multi-conversion tuner

ABSTRACT

The present invention is directed to a system and method of reducing frequency interference in a circuit when the harmonics of at least two frequencies could cause interference when such harmonics interact with each other. In one embodiment, a determination is made as to which harmonic could possibly support such interference, and based upon the determined harmonic, determining which combination within said determined harmonic is likely to cause the interference.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 10/319,118, filed Dec. 13, 2002, entitled “Systemand Method for Discovering Frequency Related Spurs in a Multi-ConversionTuner,” the disclosure of which is incorporated herein by reference. Thepresent invention is related to co-pending and commonly assigned U.S.patent application Ser. No. 08/904,693, entitled “Dual Mode Tuner forCo-Existing Digital and Analog Television Signals,” filed Aug. 1, 1997;and Ser. No. 09/572,393, entitled “Broadband Integrated Tuner,” filedMay 16, 2000, the disclosures of which are hereby incorporated herein byreference in their entirety.

TECHNICAL FIELD

The invention relates generally to frequency interference calculationsand more specifically to determining frequency related spurs in amulti-conversion tuner.

BACKGROUND

In a tuner or frequency converter (such as, for example, adual-conversion tuner), an incoming signal at frequency f_(IN) is mixedwith a signal at frequency f_(LO1) from a local oscillator (LO) toproduce a signal at an intermediate frequency f_(IF). This signal maythen mixed with a signal at frequency f_(LO2) from a second localoscillator signal to produce the desired output frequency f_(OUT), in adual conversion tuner configuration. This process is illustrated in FIG.1A, which is a portion of one example of a tuner showing how the f_(LO)signals (provided by LO 12 and 13) are mixed. Such a tuner is shown inU.S. Pat. No. 5,737,035, issued Apr. 7, 1998 hereby incorporated byreference herein. Typically, but not always, the frequency of firstlocal oscillator, e.g., LO 12, is greater than that of second localoscillator, e.g., LO 13. That is, generally f_(LO1)>f_(LO2).Accordingly, reference shall be made herein to equations in which it isassumed that f_(LO1)>f_(LO2). However, it should be appreciated that theformulae herein are applicable to situations in which f_(LO2)>f_(LO1),such as by replacing f_(LO1) with f_(LO2) and replacing f_(LO2) withf_(LO1) in situations where f_(LO2)>f_(LO1).

FIG. 1B shows a simplified diagram of two mixing stages with thefiltering omitted. These filters ultimately determine final bandwidth(f_(BW)) of the tuner, but since they do not contribute to theproduction of LO-related spurs, they are omitted from FIG. 1B.

An adverse effect of the dual conversion process is the introduction ofLO-related spurs into the tuned signal. These spurs are created bycombinations of the harmonics of the LO frequencies used (f_(LO1) andf_(LO2)).

The frequency of each of the LO-related spurs can be calculated as:f _(SPUR) =n×f ₁ −m×f ₂  (1)where n and m are integer numbers representing, respectively theharmonics of the high and low local oscillator frequencies, and f₁ andf₂ are the local oscillator frequencies (e.g., f_(LO1) and f_(LO2),respectively where f_(LO1)>f_(LO2)). If any spur generated by a givencombination of f_(LO1) and f_(LO2) falls within the output bandwidth(f_(BW)) of the converter/tuner, that spur can degrade the quality ofthe output signal. If a spur does exist within the desired outputbandwidth, the LO frequencies can be adjusted to different values toavoid the spur falling within the output band. As manufacturingprocesses produce denser and faster IC's, the number of harmonics(n_(MAX)) that must be considered continues to increase. Since thenumber of LO frequency combinations that can possibly create spurs in nharmonics is n², the amount of resources required to avoid the spursincreases dramatically as technology improves. As an example, at thetime the circuit shown in FIG. 1A was initially produced, the number ofharmonics (n) that were typically taken into consideration was 5.Currently, the number of harmonics typically taken into consideration ison the order of 15.

One reason why it is important to avoid LO spurious products is that aspur which is generated by multiples of f_(LO1) and f_(LO2) in a doubleconversion system will often have a power level which is much greaterthan the actual RF signal. Therefore, if a spur caused by a product off_(LO1) and f_(LO2) falls in the desired IF output pass band, itsamplitude (power level) may be larger than the IF output level of theoriginal desired signal, corrupting the performance of the mixer itself.

One of the fixes for this problem is that when it is known that acertain spur (such as a spur associated with two times the first LO andthree times the second LO) will fall within the output pass band, the LOfrequencies can be changed (up or down) a certain amount, which will, ineffect, still allow the circuit to tune to the desired output frequency,but the spur will be moved up or down and outside of the outputbandwidth of the tuner.

Accordingly, one method for identifying spurs falling within aparticular band, such as the tuner output band, is to look at all theharmonics of the first LO, mixed with all the harmonics of the second LOand, one by one, check off each one. Thus, if a circuit designer islooking up to the 15^(th) harmonic of the first LO and the 15^(th)harmonic of the second LO, the designer checks one times f_(LO1) (firstharmonic) and one times f_(LO2) (first harmonic) to see if there is aspur of concern. If there is no spur of concern, then the designercontinues with one times f_(LO1) (first harmonic) and two times f_(LO2)(second harmonic) to see if there is a spur of concern. If not, then theprocess continues with one times f_(LO1) (first harmonic) and threetimes f_(LO2) (third harmonic) to see if there is a spur of concern.Once all harmonics of f_(LO2) have been considered, the harmonic of thefirst LO frequency may be incremented and each harmonic of the second LOfrequency again considered. That is, the designer continues with twotimes f_(LO1) (second harmonic) and one times f_(LO2) (first harmonic)to see if there is a spur of concern, and so on. This results in n²combinations being looked at. This is a time consuming method. Evenassuming that the mathematics of how spurs are generated allows for theelimination of quite a few of the coefficients for the first and secondLO, the operation remains essentially an n² operation.

It should be appreciated that spur identification and avoidance asdiscussed above is dependent on the circuit that is being used and whichspurs might come through the chip more strongly than other spurs. It isalso dependent on the input frequency and on all the specific channelsthat might be on the input frequency. That method is also specific tothe first IF frequency and to the output frequency. Thus, for eachapplication of a circuit the chip designer generally must employ aunique program for each channel input lineup in the desired frequencyspectrum. This then implies that a different spur avoidance algorithmmust be created for every customer application, i.e., each tunerimplementation.

BRIEF SUMMARY

The present invention is directed to systems and methods of reducinginterference in a circuit resulting from harmonics of oscillatorfrequencies. In one embodiment, a determination is made as to a band orbands in which harmonics could possibly result in interference anddetermining which combination of LO frequencies result in harmonics notfalling within the determined band or bands. Preferred embodimentsleverage the fact that harmonics of a particular frequency are evenlyspaced to avoid examining all of the possible harmonics. For example,rather than calculate every harmonic and check that each calculatedharmonic does not fall within the determined band or bands, embodimentsof the present invention determine the smallest harmonics that aregreater than each edge of the determined band or bands. An interferingspur, a difference of the LO harmonics falling within the band or bands,may be determined to exist where the smallest harmonic difference for aparticular LO harmonic that is greater than a first edge of a determinedband is not equal to the smallest harmonic difference for the particularLO harmonic that is greater than a second edge of the determined band.

Accordingly, in one embodiment, the number of calculations used to findspurs that may interfere in a dual-conversion tuner is reduced bycalculating which spurs are of possible interest for each of the LOharmonics. Usually, the number of possible spurs which could possiblycause interference is reduced to only two for each LO harmonic, so theabsolute order of calculations as well as the number of calculationsbecome a number times n operation instead of a n² operation.

In one embodiment, methods of the present invention eliminate severalsets of harmonics as producing differences which cannot result in spurswithin the band or bands of interest, and thereby results in a furtherreduced set of calculations. Thus, (assuming 15 harmonics), rather thanhaving to look at 15 times 4 sets of numbers, embodiments of theinvention may only look at 5 harmonics and ignore the remaining 10. Forexample, by observing the actual value of the calculated numbers, a useror control system may stop the spur analysis without calculating allharmonic product values.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims. The novel features which are believed to be characteristic ofthe invention, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawing, in which:

FIG. 1A is a prior art dual conversion tuner;

FIG. 1B is a simplified diagram of two mixing stages, with filteringomitted, of a prior art dual conversion tuner;

FIG. 2 illustrates the bands of frequencies within which extraneoussignals such as spurs are not desirable;

FIG. 3 illustrates where the spurious signals from one LO harmonic liewith respect to the frequency bands shown in FIG. 2;

FIG. 4 illustrates how calculated frequencies fall inside or outside thecritical bands;

FIG. 5 shows the results of searching for spurs using prior art methods;

FIG. 6 shows the results of searching for spurs using concepts of thepresent invention;

FIG. 7A shows one embodiment of a method of practicing concepts of thepresent invention;

FIG. 7B shows a variation of a method of practicing concepts of thepresent invention of FIG. 7A; and

FIG. 8 shows one embodiment of a system using the concepts of thepresent invention.

DETAILED DESCRIPTION

Equation (1) (above) is the starting point for calculation of a spur. Asshown, n is a harmonic (1, 2, 3, etc.) of f₁, e.g., f_(L01), and m is aharmonic of f₂, e.g., f_(L02). The spur equation then is the product ofn f_(L01) minus the product of m f_(L02).

There is a practical limit to how many harmonics (n, m) are capable ofcausing interference, e.g., due to the signal level of the spur and/orthe frequency of the spur. The number of harmonics which are candidatesfor producing interfering spurs for any particular tuner circuit dependsupon such factors as how fast the process used to fabricate the tuner isand/or how high a frequency the tuner is designed to handle. Fasterprocesses generally suggest larger harmonics should be considered.However, spurs which fall outside the IF output pass band are typicallynot of concern.

Different circuits, and different applications, have different passbands. One such pass band, such as often utilized in television signalprocessing, is 44 MHz. Accordingly, in such circuits there should besome bandwidth around the output center frequency, e.g., 6 MHz aroundthe 44 MHz video signal pass band, which should not have spurs. Forexample, 44 plus 3 megahertz and 44 minus 3 megahertz may define a rangewhich should be free of LO spurs because that is where a demodulator islooking for a clean signal to process and use. Based on this example, asystem or system designer looks for integer harmonics of the tuner LOfrequencies that will generate spurs in the range of 41 MHz to 47 MHz,and the image band −41 MHz to −47 MHz. As discussed, it is currentpractice to check as many as 15 harmonics, thus n and m range from 1-15.

FIG. 2 illustrates the output band 20 (and its image 21) of aconverter/tuner. Any f_(SPUR) that falls within the shaded ranges is ofconcern and most likely undesirable.

FIG. 3 shows a graphical representation of the spurs (spurs 31-36) thatresult for a specific harmonic (n) of a first LO frequency (e.g.,f₁=f_(LO1)) and six harmonics (m=−1, m=−2, m=−3, m=−4, m=−5, m=−6) of asecond LO frequency (e.g. f₂=f_(LO2)). These spurs may be calculatedusing Equation (1). In the example shown in FIG. 3, there are nointerfering spurs associated with the n^(th) harmonic of f₁ (f_(LO1))when combined with the 6 illustrated harmonics of f₂ (f_(LO2)).Moreover, it can be seen that many of the spurs are displaced infrequency far enough from the bands of interest (output band 20 andimage 21) so as to present no real issue with respect to interference,and therefore needlessly consume computing resources in theircalculation.

The fact that the LO-related spurs are evenly spaced may be usedaccording to the embodiments of the invention to avoid having to examineall of the possible harmonics and their spur. Rather than calculatespurs for every harmonic and then check to be sure that the calculatedspur does not fall within the output bandwidth (or its image), it ispossible to determine for each/any particular harmonic of f₁ thesmallest harmonic of f₂ that produces a spur greater than each edge ofbands 20 and 21, (edges A, B, C, and D). If the smallest harmonic of f₂which results in a spur larger than one edge of a band (e.g., edge C ofband 20) is not equal to the smallest harmonic of f₂ which results in aspur larger than the other edge of the band (e.g., edge D of band 20),then at least one spur falls between the two band edges, that is, a spurexists in the band.

The frequencies of the output band edges discussed above may berepresented as (where f_(BW) is the bandwidth of the output band):A=−f _(OUT) −f _(BW)/2  (2)B=−f _(OUT) +f _(BW)/2  (3)C=f _(OUT) −f _(BW)/2  (4)D=f _(OUT) +f _(BW)/2  (5)

For a given harmonic n of f₁, the exact multiple of f₂ that isassociated with a spur coinciding with each band edge can be calculated.The multiple that corresponds for point A, for example, can becalculated as (substituting A as defined in equation (2) for f_(SPUR) ofequation (1) and solving for m)m _(A)=(n×f ₁ +f _(OUT)+(f _(BW)÷2))÷f₂  (6)Since we are only interested in integer values of m, we can apply thefloor function (└m_(A)┘) to this value to, determine the smallestinteger harmonic of f₂ that is associated with a spur greater than bandedge A. The equations to determine such an integer value for each of theband edges are:m _(A)=└(n×f ₁ −A)÷f ₂┘  (7)m _(B)=└(n×f ₁ −B)÷f ₂┘  (8)m _(C)=└(n×f ₁ −C)÷f ₂┘  (9)m _(D)=└(n×f ₁ −D)÷f ₂┘  (10)The above 4 harmonics of f₂ may be analyzed according to the presentinvention to determine if spurs resulting from harmonics of f₂ and aparticular harmonic (n) of f₁ correspond to band 20 or 21. Ifm_(A)≠M_(B) or m_(C)≠m_(D) then an LO-related spur falls within theoutput bandwidth of the tuner.

Assume by way of example, that m_(A) before applying the floor functioncomputes to be a number, such as 5.3, and m_(B) computes to be anothernumber, such as 5.2, when considering the 2^(nd) harmonic of f₁. Takingthe floor of 5.3 and 5.2, yields 5 for both m_(A) associated with edge Aand m_(B) associated with edge B. Accordingly, analysis of m_(A) andm_(B) provided according to embodiments of the invention may concludethat no spur is present in band 21 associated with the 2^(nd) harmonicof f₁, in combination with the harmonics of f₂. This can be seengraphically in FIG. 4, where the 5^(th) harmonic of f₂ when combinedwith the 2^(nd) harmonic off produces a spur (spur 45) which is both thesmallest harmonic of f₂ less than A and B and, therefore, does not showup between points A and B. Saying this another way, there was no wholenumber transition between the spurs associated with the edges of theband, so no spurs will show up between points A and B.

Another way to look at the spur analysis described above would be to saythat the A, B, C and D edge values are known for any particular system.Since back edge frequency A is known, and n (the harmonic of f₁) isfixed for a particular set of spurs, a value of m (the harmonic of f₂)which would cause a spur to happen at A can be calculated. In thiscalculation, m need not be a whole number, or an integer number, butrather an exact calculation of what that number is. Equation (6)calculates m for frequency A (for the above example, m was 5.3).Equation (7) takes the floor of that calculation to arrive at an integervalue since a harmonic is present only at integer multiples of aparticular frequency (in the example applying the floor function to 5.3provides 5). Because there is no transition in the calculated harmonicvalues for band edge A and band edge B (i.e., m_(A)=m_(B)) there can beno LO-related spur within the band defined by band edge A and band edgeB.

However, in FIG. 4 by way of example, it can be seen that the spurrelated to the second harmonic of f₁ and the third harmonic of f₂ fallswithin band 20 of concern. When the equations above are used tocalculate the smallest harmonics having spurs greater than the bandedges C and D, m_(C)=3 and m_(D)=2, indicating a transition in thecalculation harmonic values for edge band C and edge band D (i.e.,m_(C)≠m_(D)). This indicates that a spur exists in band 20, even withoutcalculating the frequency of the spur. Accordingly, it can readily beappreciated that embodiments of the present invention provide anexpedient technique for recognizing combinations of LO frequencieshaving undesired spurs associated therewith.

Moreover, the floor function used according to preferred embodiments ofthe invention allows for a fixed point integer operation since it is notimportant to the spur determination what the fractional part is.Accordingly, the divide according to embodiments of the invention, maybe implemented as an integer divide, which is a function that most CPUsprovide and therefore is readily implemented. It should be appreciatedthat, although the example above discussed calculations with respect toa single particular harmonic of the first frequency (f₁), embodiments ofthe present invention perform such calculations for a series of firstfrequency harmonics. For example, n may be selected to be 1 (1^(st)harmonic of f₁) and the above calculations and analysis of m_(A), m_(B),m_(C), and m_(D) performed. If the presence of undesired spurs is notidentified, n may be incremented (such as 2, corresponding to a 2^(nd)harmonic of f₁) and the above calculation and analysis of m_(A), m_(B),m_(C), and m_(D) performed, and so on.

Embodiments of the present invention may further optimize thecalculations and analysis performed in determining the presence ofspurs. For example, some embodiments of the invention operate tocalculate m_(A) and m_(D) to determine if a transition (spur) happensbetween them as a threshold determination for further analysis of a setof harmonics. If a spur does occur between distal band edges A and D,then the system may proceed to calculate proximal band edges B and C forfurther analysis. For example, if the transition occurred betweenproximal edges B and C it could not occur between band edges A and B(band 21) or band edges C and D (band 20). Note that the charts shown inFIGS. 3 and 4 are for discussion purposes only and in reality thedistance between A and D is in the order of 70-90 megahertz for a videosignal processing system, while the distance between 1 times f₂ and 2times f₂ is around 1200 megahertz for such a video signal processingsystem. Accordingly, the chance of a spur actually falling between A andD is slight (about 7%). Thus, this embodiment can shorten the number ofcomputations (reducing the number of computations by half about 93% ofthe time) by initially looking at just points A and D.

It should be appreciated that, although embodiments have been describedherein with reference to use of the floor function, it is also possibleto use the ceiling function for the same purpose. Such embodiments ofthe invention calculate the largest harmonic that is less than the bandedge using the ceiling function, with the comparisons between the m_(X)values remaining as described above.

By using the concepts taught herein, one need only calculate, at most,four values for each n×f₁ value, regardless of how many harmonics(n_(MAX)) are to be considered. In addition, since the value of n×f₁increases as n increases, if the values of n are considered inincreasing order, when the calculated value of m_(D), the largest m_(X)quantity, equals or exceeds n_(MAX) (m_(D)≧n_(MAX)) then there are nomore combinations of n≦n_(MAX) and m≦n_(MAX) which could produceinterfering spurs. It is, thus, not necessary to examine any additionalharmonics of f₁.

As an example of the improved efficiency of this algorithm, consider atuner with an output frequency of 44 MHz, where the local oscillatorsare tuned to f_(LO1)=f₁=1289 MHz and f_(LO2)=f₂=1176 MHz, and the outputbandwidth is 6 MHz. Thus, the output frequency of interest is 41 MHz to47 MHz. In this example, there is a spur (46 MHz) resulting from thetenth harmonic of f_(LO1) (10×1289) and the eleventh harmonic of f_(LO2)(11×1176) that falls within the output bandwidth(10×1289−11×1176=−46.0).

Existing spur-avoidance routines can require the calculation of all n²possible spur frequencies to compare the value of each spur to theoutput bandwidth as in equation (11) below. When this equation is true,there is a spur within the output band.

$\begin{matrix}{{\left. {f_{OUT} -} \middle| f_{SPUR} \right.} \leq \frac{f_{BW}}{2}} & (11)\end{matrix}$Searching n=11, and as many as n=15, harmonics is not uncommon.Accordingly, it can readily be appreciated that analysis using existingspur-avoidance routines can be not only be time consuming but processorintensive.

FIG. 5 shows the results of searching for the spur described above,using common methods and checking for n=15 harmonics. When any spur isdetected, no further calculation is necessary. This method requirescalculating 146 spur frequencies to be compared with the outputfrequency. Indeed, when no spur is present, the usual situation, it maybe necessary to examine all n² frequencies, which in this case is 225calculations.

FIG. 6 shows the results of determining the smallest harmonics greaterthan the band edges to detect the spur. Only 38 values are computed inthe illustrated example before a spur is detected between m_(A) andm_(B) associated with the 10^(th) harmonic of f_(LO1) and the 11^(th)harmonic of f_(LO2). If there is no spur in the band at all, a maximumof 4n (where n, is the number of harmonics to be checked) values arecalculated. In this case 60 calculations would be the maximum number.However, since the calculated m_(X) value reflects the number of f_(LO2)harmonic larger than the band edge and the m_(X) values only increase,for higher vales of n, the scan can be terminated as soon as the valueof m_(D) matches or exceeds n_(MAX), meaning that it will seldom requirethe calculation of all 60 values.

Additionally, if the values of m_(A) and m_(D) are computed first andm_(A)=m_(D), then it is not necessary to calculate the m_(B) and m_(C)values (since they must also be the same), which reduces the number ofcalculations in FIG. 6 from 38 to 21.

Additional optimization to the above algorithm may be obtained bydetermining the first harmonic of f₁ that can possibly contribute to aninterfering spur (n_(O)), allowing any calculations for lower harmonicsof f₁ to be eliminated. In implementing embodiments of the inventionincluding such further optimization, local oscillator frequencies (f₁and f₂) are selected such that the difference between the LO frequencies(f₁−f₂) is greater than the highest frequency in the output band(f_(OUT)+f_(BW)/2). Stated another way:

$\begin{matrix}{{{{n \times f_{1}} - {m \times f_{2}}} > {f_{OUT} + \frac{f_{BW}}{2}}},\mspace{31mu}{{{where}\mspace{14mu} n} = {m = 1}}} & (12)\end{matrix}$Since f₁>f₂, and therefore, n×f₁>n×f₂, it follows that equation (12) isalso true for all values of m that are less than or equal to n.Therefore

$\begin{matrix}{{{{n \times f_{1}} - {m \times f_{2}}} > {f_{OUT} + \frac{f_{BW}}{2}}},\mspace{31mu}{{{for}\mspace{14mu}{all}\mspace{14mu} n} \geq m}} & (13)\end{matrix}$This means that no LO-related spurs will be created in the outputbandwidth when m≦n, or conversely, all spurs will occur when m>n. Thefirst possible spur cannot occur until (at least) m=n+1.

The above knowledge can be combined with the fact that, by definition,all interfering spurs must be larger than band edge A (see FIG. 3), todetermine the lowest harmonic of f₁ which needs to be examined (n_(O)).Combining the equation for the first possible spur (from equation (1))and the value of band edge A (equation (2)) produces the followingrelationship describing the first harmonic (n_(O)) of interest:

$\begin{matrix}{{{n \times f_{1}} - {\left( {n + 1} \right) \times f_{2}}} > {{- f_{out}} - \frac{f_{BW}}{2}}} & (14)\end{matrix}$Solving this equation for n yields the lowest harmonic of f₁ that canproduce spurs:

$\begin{matrix}{n > \frac{f_{2} - f_{OUT} - \frac{f_{BW}}{2}}{f_{1} - f_{2}}} & (15)\end{matrix}$Since only integer harmonics are meaningful and n is the minimumharmonic of f₁ that may produce a spur, the ceiling function (┌ ┐) canbe applied to the value of n to determine n_(O):

$\begin{matrix}{n_{O} = \left\lceil \frac{f_{2} + A}{f_{1} - f_{2}} \right\rceil} & (16)\end{matrix}$In the example above, where f_(LO1)=1289 MHz and f_(LO2)=1176 MHz, thefirst harmonic of interest is n=10. When determining the smallest spurlarger than each band edge according to embodiments of the presentinvention and starting with the calculated minimum harmonic of f₁ asdetermined above, the presence of the interfering spur of the example ofFIG. 6 determined by the two additional calculations, e.g., m_(A) andm_(D), it is only necessary to calculate 2 values to detect theexistence of the interfering spur.

Summarizing the above discussion, if the two LO frequencies are fartherapart than the output bandwidth, which is nearly always the case, thenthe only spur which could occur is when n (the multiplier of the LO1harmonic) is a larger number than the LO2 harmonic, or where n isgreater than m. This is discussed with respect to equation (12) andequation (13). By accepting that fact and looking back at FIGS. 3 and 4,it can be seen that after frequency A is passed (whatever that frequencyis) there cannot be any more spurs resulting in bands 20 and 21 as allthe spurs will be larger in frequency than band edge A. Equations (14),(15), and (16) address this fact and allow for the determination of thefirst harmonic of LO1 that can produce a spur. In all the other casesLO1 is small enough such that when even the very first LO2 issubtracted, the result will be less than the frequency of band edge A,which is the lowest frequency we are interested in. Taking this all intoconsideration, equation (16) emerges which produces an initial LO1harmonic of interest.

FIG. 7A shows the steps of a method for determining if particular LOfrequencies result in undesired spurs (spurs in an output band) arepresent according to an embodiment of the present invention. Step 701determines the band edges for the tuner. It should be appreciated that,in most situations, band edge numbers are constant values that do notchange, so they only need to be calculated one time for a particularimplementation. Step 702 calculates no, the first f₁ harmonic ofinterest, to thereby optimize the number of calculations made indetermining the presence of an undesired spur. According to theillustrated embodiment, the harmonic of f₁ for spur analysis (n) is setto the calculated first harmonic of interest (n_(O)).

Step 703 calculates the smallest harmonic of f₂ that is associated witha spur greater than band edge A (m_(A)) and the smallest harmonic of f₂that is associated with a spur greater than band edge D (m_(D)) for theselected value of n. At step 704 a comparison is made with respect tothe calculated harmonics to determine if it is possible that a spur ispresent in the bands of interest. If m_(A)=m_(D) then no spur can bepresent in the bands of interest so processing according to theillustrated embodiment proceeds to step 709. If m_(A)≠m_(D) a spurexists between distal band edges A and D and, therefore, may be presentin the bands of interest. Accordingly, processing according to theillustrated embodiment proceeds to step 705 for further spur analysis.

Step 705 calculates the smallest harmonic of f₂ that is associated witha spur greater than band edge B for the selected value of n. At step 706a comparison is made with respect to the calculated harmonics todetermine if a spur is present in a first band of interest (the banddefined by band edge A and band edge B). If m_(A)≠m_(B) then it isdetermined that an undesirable spur exists and spur determinationprocessing with respect to the pair of LO frequencies (f₁ and f₂) stops.If m_(A)=m_(B) then no spur is present in the first band of interest,but the spur between band edges A and D (as determined at step 704) maybe present in a second band of interest (the band defined by band edge Cand band edge D). Accordingly, processing according to the illustratedembodiment proceeds to step 707 for further spur analysis.

Step 707 calculates the smallest harmonic of f₂ that is associated witha spur greater than band edge C for the selected value of n. At step 708a comparison is made with respect to the calculated harmonics todetermine if a spur is present in the second band of interest. Ifm_(C)≠m_(D) then it is determined that an undesirable spur exists andspur determination processing with respect to the pair of LO frequencies(f₁ and f₂) stops. If m_(C)=m_(D) then no spur is present in the secondband of interest and, therefore, the spur between band edges A and D (asdetermined at step 704) must be out of band between band edges B and C.Accordingly, processing according to the illustrated embodiment proceedsto step 709.

At step 709 a determination is made as to whether the smallest harmonicof f₂ that is associated with a spur greater than band edge D (m_(D))for the selected value of n is greater than or equal to a maximumharmonic (n_(MAX)) of f₁ for analysis, e.g., n_(MAX) may be establishedat 15 for relatively fast tuner circuits. If the smallest harmonic of f₂corresponding to band edge D (m_(D)) for the selected value of n isgreater than or equal to the maximum harmonic, then no further spuranalysis is performed according to the illustrated embodiment as thepair of LO frequencies (f₁ and f₂) do not have spurs which are presentin the bands of interest. If the smallest harmonic of f₂ correspondingto band edge D (m_(D)) for the selected value of n is less than themaximum harmonic, then processing proceeds to step 710 for spur analysiswith respect to a next harmonic of f₁ (a next selected value of n).

At step 710 the harmonic of f₁ (n) is incremented. Thereafter, steps703-710 are repeated with respect to the next set of spurs.

FIG. 7B shows a variation to the embodiment of FIG. 7A in which thenumber of steps utilized in determining LOs having interfering spursassociated therewith may be reduced. The steps of FIG. 7B would replacesteps 703-710 of FIG. 7A, providing a determination with respect tom_(D) reaching a maximum harmonic (n_(MAX)) earlier in the algorithm.Although the steps of FIG. 7B provide substantially the samecalculations and comparisons as those of FIG. 7A (it being appreciatedthat features of step 703 in FIG. 7A are divided among steps 703 a and703 b of FIG. 7B), calculations and/or comparisons of several steps maybe avoided according to the embodiment of FIG. 7B due to the earlierdetermination with respect to the maximum harmonic.

FIG. 8 shows device 80 in which tuner 81 in conjunction with demodulator83 uses the concepts taught herein to adjust LO frequencies forselectively tuning signals from a plurality of frequency divisionchannels received via antenna 87 and/or cable system 88, via input 86,for use by using device 82. For example, using device 82 may comprise atelevision set, personal computer, radio, or other device receivingdigital and/or analogue signals via tuner 81 and demodulator 33.Accordingly tuner 81 and demodulator 83 may comprise a system such as aset-top cable box, cable modem, video tuner expansion card, etcetera, ormay comprise a portion of using device 82.

Adjustment of LO frequencies used by tuner 81 may be to select aparticular carrier frequency for conversion to a particular output. IFof tuner 81 for further processing of the signal, such as by demodulator83. This adjustment could be controlled by processor and/or memory ontuner 80 (not shown) or tuner 81 could receive control signals (orreceive the actual LO frequencies) from an external source, such asdemodulator 83 having contained therein processor 84 and memory 85.Communications can occur, for example, over connection 801 which istypically a wire connection but could be wireless.

Tuner 81 would typically receive input channels from antenna 87 and/orcable system 88 over input 86. Demodulator 83 would accept the output oftuner 81 (and/or provide input to tuner 81) and would provide signals tousing device 82 integral therewith or separate therefrom.

As a new (different) carrier frequency is selected (a new channel isselected), the LO frequencies of tuner 81 are adjusted to turn to thenewly selected carrier frequency. However, these LO frequencies must becarefully chosen to avoid spurious signals appearing in the output IF oftuner 81. This is, although a number of LO frequencies may provideconversion of a signal from a particular RF carrier frequency to aparticular IF frequency, many such LO frequency combinations will havespurs associated therewith which also fall within the IF frequencyoutput band. Accordingly, before implementation of a particular LOfrequency combination for tuning a desired signal by tuner 81, theLO-related spurs are analyzed for undesired spurs. In device 80 of theillustrated embodiment, selection of LO frequencies and the associatedspur analysis is done dynamically, such as at the time of channelselection. Accordingly, the concepts of the present invention areemployed to minimize delay in tuning to selected channels.

One advantage of systems and methods of the present invention is timesavings for alignment, when a tuner is used over a wide range offrequencies. In such a situation it is important to find the LO spursquickly with as few calculations as possible. In the example of a settop box or a cable box, these calculations are made every time a channelis changed. In such a situation, when the button for a channel change ispushed, the system goes through the software looking for the spurs forthat new channel and must calculate what the LOs should be that willtune that channel and yield no spurs on the output. Since thecalculations must be the same for all situations (because ofcertification processes) it is not easy to change calculationprocedures. Also, since the same tuner system serves different markets,having different frequencies, it is difficult to change calculationprocedures on a market by market basis. The concepts taught herein allowa tuner to be certified once and have that same tuner with any channellineup. Also, since this procedure could run on an embedded processor,such as processor 84 (FIG. 8) having limited memory capacity, withlimited speed, such as memory 85, it is important to reduce calculationsfor spurs as discussed herein.

Moreover, systems and methods of the present invention are also helpfulin applications that do a lot of channel changing, for example, sweepingof channels in a cable modem, such as shown in FIG. 8. When a cablemodem starts, it normally sweeps through the channel spectrum to findthe signal that it is looking for. This tuning of the tuner may be donehundreds of times at different frequencies to find the correct channel.Some cable modems actually tune through every MHz looking for morechannels which could send information downstream to the cable modem.These modems typically do not know ahead of time what the channel lineupis. When the modem finds a proper channel it uses that channel andobtains information from the head end indicating where each channelresides.

Note that while the embodiments discuss local oscillator frequencies,the inventive concepts would be applicable to any frequency interferencesensitive circuit or system where the harmonics of frequencies could addspurs (or extraneous frequencies) into a circuit at specificfrequencies.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

1. A method for assisting in the determination of non-interfering localoscillator (LO) frequencies (f₁, and f₂) in a dual conversion tunersystem, said method comprising: determining band edges A, B, C, and D ofa system output band and its image using the formulaeA=−f _(OUT) −f _(BW)/2B=−f _(OUT) +f _(BW)/2C=f _(OUT) −f _(BW)/2D=f _(OUT) +f _(BW)/2,  where f_(OUT) is the system output pass bandcenter frequency and f_(BW) is the bandwidth of the system output passband; for a selected harmonic of f₁, calculating a smallest harmonic off₂ greater than band edge A, m_(A), and a smallest harmonic of f₂greater than band edge D, m_(D), using the formulaem _(A)=└(n×f ₁ −A)÷f ₂┘m _(D)=└(n×f ₁ −D)÷f ₂┘,  where n is the selected harmonic of f₁; anddetermining if m_(A)=m_(D).
 2. The method of claim 1, furthercomprising: if m_(A)≠m_(D), calculating m_(B) using the formulam _(B)=└(n×f ₁ −B)÷f ₂┘; and determining if m_(A)≠m_(B).
 3. The methodof claim 2, further comprising: if m_(A)≠m_(B), concluding thatfrequencies f₁ and f₂ result in an interfering spur.
 4. The method ofclaim 2, further comprising: if m_(A)=m_(B), calculating m_(C) using theformulam _(C)=└(n×f ₁ −C)÷f ₂┘; and determining if m_(C)≠m_(D).
 5. The methodof claim 4, further comprising: if m_(C)≠m_(D), concluding thatfrequencies f₁ and f₂ result in an interfering spur.
 6. The method ofclaim 4, further comprising: if m_(C)=m_(D), determining ifM_(D)≧n_(MAX), where n_(MAX) is a maximum harmonic to be considered; andif M_(D)≧n_(MAX), concluding for the selected harmonic of f₁ thatfrequencies f₁ and f₂ do not result in an interfering spur.
 7. Themethod of claim 6, further comprising: if m_(D)<n_(MAX), selecting anext harmonic of f₁ and repeating, for said next selected harmonic off₁, said calculating a smallest harmonic of f₂ greater than band edge A,m_(A), and a smallest harmonic of f₂ greater than band edge D, m_(D),and determining if m_(A)=m_(D).
 8. The method of claim 1, furthercomprising: calculating a first f₁ harmonic (n_(O)) using the formula${n_{O} = \left\lceil \frac{f_{2} + A}{f_{1} - f_{2}} \right\rceil},$where n₀ is a limit on the harmonics of f₁ considered according to themethod.
 9. A system for assisting in the determination ofnon-interfering local oscillator (LO) frequencies (f₁ and f₂) in a dualconversion tuner system, said system comprising: means for determiningband edges A, B, C, and D of a system output band and its image usingthe formulaeA=−f _(OUT) −f _(BW)/2B=−f _(OUT) +f _(BW)/2C=f _(OUT) −f _(BW)/2D=f _(OUT) +f _(BW)/2,  where f_(OUT) is the system output pass bandcenter frequency and f_(BW) is the bandwidth of the system output passband; means for calculating, for a selected harmonic of f₁, a smallestharmonic of f₂ greater than band edge A, m_(A), and a smallest harmonicof f₂ greater than band edge D, m_(D), using the formulaem _(A)=└(n×f ₁ −A)÷f ₂┘m _(D)=└(n×f ₁ −D)÷f ₂┘  where n is the selected harmonic of f₁; andmeans for determining if m_(A)=m_(D).
 10. The system of claim 9, furthercomprising: means for calculating, if m_(A)≠m_(D), m_(B) using theformulam _(B)=└(n×f ₁ −B)÷f ₂┘; and means for determining if m_(A)≠m_(B). 11.The system of claim 10, further comprising: means for concluding if,m_(A)≠m_(B), that frequencies f₁ and f₂ result in an interfering spur.12. The system of claim 10, further comprising: means for calculating,if m_(A)=m_(B), m_(C) using the formulam _(C)=└(n×f ₁ −C)÷f ₂┘; and means for determining if m_(C)≠m_(D). 13.The system of claim 12, further comprising: means for concluding if,m_(C)≠m_(D), that frequencies f₁ and f₂ result in an interfering spur.14. The system of claim 12, further comprising: means for determining,if m_(C)=m_(D), if m_(D)≧n_(MAX), where n_(MAX) is a maximum harmonic tobe considered; and means for concluding, if m_(D)≧n_(MAX), for theselected harmonic of f₁ that frequencies f₁ and f₂ do not result in aninterfering spur.
 15. The system of claim 14, further comprising: meansfor selecting, if m_(D)<n_(MAX), a next harmonic of f₁ and repeating,for said next selected harmonic of f₁, said calculating a smallestharmonic of f₂ greater than band edge A, m_(A), and a smallest harmonicof f₂ greater than band edge D, m_(D), and determining if m_(A)=m_(D).16. The system of claim 9, further comprising: means for calculating afirst f₁ harmonic (n_(O)) using the formula${n_{O} = \left\lceil \frac{f_{2} + A}{f_{1} - f_{2}} \right\rceil},$where n₀ is a limit on the harmonics of f₁ considered.
 17. A method forassisting in the determination of non-interfering local oscillator (LO)frequencies (f₁ and f₂) in a dual conversion tuner system, said methodcomprising: determining band edges A, B, C, and D of a system outputband and its image, wherein band edge A comprises a lowest frequencyband edge of band edges A, B, C, and D and band edge D comprises ahighest frequency band edge of band edges A, B, C, and D; for a selectedharmonic of f₁, calculating a smallest harmonic of f₂ greater than bandedge A, m_(A), and a smallest harmonic of f₂ greater than band edge D,m_(D); and determining if m_(A)=m_(D).
 18. The method of claim 17,wherein said determining band edges A, B, C, and D uses the formulaeA=−f _(OUT) −f _(BW)/2B=−f _(OUT) +f _(BW)/2C=f _(OUT) −f _(BW)/2D=f _(OUT) +f _(BW)/2,  where f_(OUT) is the system output pass bandcenter frequency and f_(BW) is the bandwidth of the system output passband.
 19. The method of claim 17, wherein said calculating said smallestharmonic of f₂ greater than band edge A, m_(A), and said smallestharmonic of f₂ greater than band edge D, m_(D), uses the formulaem _(A)=└(n×f ₁ −A)÷f ₂┘m _(D)=└(n×f ₁ −D)÷f ₂┘, where n is the selected harmonic of f₁.
 20. Themethod of claim 19, further comprising: if m_(A)≠m_(D), calculatingm_(B) using the formulam _(B)=└(n×f ₁ −B)÷f ₂┘; determining if m_(A)≠m_(B); and if m_(A)≠m_(B),concluding that frequencies f₁ and f₂ result in an interfering spur.